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Lecture 36
Quasars and Active Galaxies
[not on exam]
January 16a, 2014
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Active Galaxies
• Emit much more energy than average
galaxies (100,000 times in some cases)
• More seen at larger distances (past)
• Galaxies radiate light at different
wavelengths than normal galaxies
– normal -- most light in visible wavelengths
– active galaxy -- most light in radio and infrared
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Quasars
• 1960 – Allan Sandage
– Observed strong radio source 3C 48.
– Found star-like object at source with
unidentified emission lines.
• Later others were found.
– 3C 273: Emission lines were
extremely red-shifted hydrogen lines
– Source was 600 Mpc away (2 billion
light years)!
• Quasi-stellar objects = quasars
3C 273
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Quasars
• Quasars are very bright sources and are at
distances very luminous
– Can be 1000’s of times more luminous than the
Milky Way.
– Do not emit much blackbody radiation.
• Now know of ~10,000 quasars.
– Most have distances > 1000 Mpc (3 billion
light years)
– No nearby quasars = no recent quasars
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Quasars are in Distant Galaxies
• Host galaxies often appear distorted and
have companion galaxies.
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Active Galaxy – a
galaxy that is much
more luminous than
ordinary galaxies.
Two kinds – Seyfert
galaxies and radio
galaxies
Peculiar Galaxy –
any galaxy that
appears to be
blowing apart
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Seyfert Galaxies
•
•
•
•
Spiral galaxy with very bright nucleus
Less luminous than quasars
Most are seen at large distances
Most emission comes from small region in
the active galactic nucleus (bright core)
– Center ~10,000 times brighter than core of MW
• Luminosity fluctuates with time
• Most energy emitted in radio and IR  not
stars
• Broad, strong emission lines  not stars
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Seyfert Galaxy
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Seyfert Galaxy
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Radio Galaxies
• Core-Halo
galaxies
– Small central
core
– Halo of radio
emission
around it
Figure 24.24, Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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• Lobe Radio Galaxies
– Small nucleus
– Large radio lobes
extend from galaxy
– Lobes possibly
material ejected from
central galaxy
Galaxy Cygnus A appears to really be
two galaxies colliding. It displays huge
radio-emitting lobes on either side of
the optical image.
Figure 24.23, Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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• The two types radio galaxies are likely the same
object, just seen from different angles
Figure 24.25,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Active Galaxy Engine
• Possible Theory: Supermassive Black Holes
– Supermassive (>1,000,000 suns) black hole is
in core of galaxy
– Infalling gas and stars form accretion disk
– Material heated by collisions in disk and
interacts with magnetic field
– Emits large amounts of radiation
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Figure 24.31, Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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• Disks have been observed near centers of
some of these galaxies
Figure 24.32, Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall